Industrial plastic processing machines utilize multi-phase electricity as an energy source to generate heat for melting and/or maintaining plastic in a molten state. The electricity is converted into thermal energy by electrical resistance heating elements which are located on a molten plastic vessel associated with the molding machine. The temperature of the molten plastic is controlled by selectively turning on and off the electrical power supply to the electrical resistance heating elements. In conventional industrial plastic processing machines the electrical power is switched on and off by electromagnetic relays having mechanical contacts. Each phase of the electrical supply is switched by individual pairs of contacts. Heat control using electromagnetic relays is capable of reliably maintaining molten plastic at a temperature range of minus zero, plus 7.degree. F. Operating in such a temperature range, the relays are cycled approximately once per minute. To realize more accurate temperature control, the relays must be cycled on and off more frequently. For example, minus zero, plus 1.degree. F. temperature regulation may be realized if the relays are cycled approximately once per second. Such high frequency relay cycling raises the operating temperature of the relay contacts and reduces the life of the contacts. Failure of the contacts results in a loss of conduction of electrical energy. Typically, contact failure occurs in individual phases and not in all phases at one time. With conventional electromagnetic equipment, loss of conduction of a single phase can be discovered only by individual examination of each contact. When a phase loss condition occurs, the remaining phases must pick up the heating load as required to meet thermostatic demand conditions. As a result, the molten plastic material surrounded by the operating electrical resistance elements is overheated and the molten plastic surrounded by the non-operating electrical resistance elements is underheated. Such uneven heating of the plastic reduces the structural and dimensional quality of products formed therefrom. In addition, the remaining operating phases must handle a greater load. This added load shortens the life of the remaining phases.
Efforts to replace the conventional electromagnetic relays with electronic components which are capable of high frequency switching for extended periods of time are limited by the fact that the space available for retrofit mounting of such a circuit within the conventional electrical panel is extremely restricted. Locating the circuit in a remote panel is effective but requires an extra panel box and remote wiring. Many remote circuits also utilize electromagnetic contacts to engage the electronic switching components in each phase and are susceptible to mechanical failure and possible phase loss operation.
The present invention features a heat sink structure on which the electronic components required for switching may be mounted in an arrangement having the same dimensions as that of the conventional electromagnetic relay. Therefore, no additional remote panel is required. The present invention includes an electronic circuit having two pair of inverse parallel connected silicon control rectifiers connected in two phases and a silicon control rectifier switching circuit. The switching circuit includes a first portion which converts the conventional logic signal into one acceptable to the SCR's. The second portion of the switching circuit includes a photo isolation coupling and a zero cross switching circuit in association with each pair of inverse parallel connected silicon control rectifiers.
The first portion of the switching circuit is connected to the photo isolation couplings of the second portion in a parallel circuit arrangement. Such a circuit arrangement disables the entire circuit if the first portion of the triggering circuit or either photo isolation coupling fails thereby preventing operation in a lost phase condition.
The photo isolation coupling prevents any electrical disturbance from being communicated from the electrical switching portion to the logic source. The zero cross switching circuit holds the logic signal until the instantaneous value of the amplitude of the supply voltage in the respective phase is zero and then releases the signal to switch the respective silicon control rectifier. Switching the silicon control rectifiers as the power supply voltage crosses zero minimizes the strain thereby maximizing the life of the silicon control rectifiers.
Additional objects and advantages of the present invention will become apparent by reading the detailed description of the preferred embodiment which makes reference to the following set of drawings.